Existing door frames are commonly constructed of hollow U-shaped aluminum extrusion profiles wrapped in plastic. With an application temperature of −15° F. in the refrigerator, and 75° F. and 55% relative humidity (RH) in the room, the thermal insulation of this construction is insufficient for the exterior to stay above the ambient dewpoint of 58° F. As a consequence, this frame requires about 100 watts/door of anti-sweat heaters to prevent exterior condensation in typical applications.
Earlier attempts to improve thermal resistance of the frame involved using fiberglass pultrusion profiles stuffed with extruded polystyrene foam insulation. This design resulted in a frame which requires about 50 watts/door of anti-sweat to prevent exterior condensation. Fiberglass pultrusion was selected to provide reduced thermal resistance versus aluminum. For example, the thermal conductivity of aluminum is 273,000 mW/m-K versus fiberglass pultrusion at 577 mW/m-K, versus rigid PVC at 170 mW/m-K. Earlier frames used aluminum or fiberglass materials for their relatively high structural strength and low coefficient of thermal expansion (CTE). While PVC provides superior thermal resistance, it lacks sufficient structure and has a high CTE which can be detrimental in freezer applications. A typical freezer door frame application can see a temperature differential of about 90° F. (75° F. outside to −15° F. inside). With a high CTE, the unrestricted shrink can be significant over a typical 153″ 5-door frameset length. For example, the CTE of aluminum, fiberglass, and rigid PVC are 12.8, 4.72, and 28.9 in/in ° F., respectively. This results in an unrestricted shrink over 153″ of 0.176″, 0.065″, and 0.398″, respectively. This large shrink for PVC may cause it to crack when screwed to a wood or steel freezer opening.
Earlier frames also suffer from the challenges of providing a structural joint without compromising thermal insulating performance. Some manufacturers weld the aluminum jambs to the header and sill members, and rivet the mullions in place using steel brackets. Foam rubber inserts are added to reduce infiltration air in these mechanically constructed corners. Other manufacturers use molded plastic corner and mullion brackets to screw the frame together. The relatively higher thermal conductivity of these molded plastic bracket result in these locations being the coldest, and thus the first places to experience external condensation.